1. Field of Invention
The present invention relates to an apparatus for X-ray photography, and in particular, to an apparatus for X-ray photography that can keep an X-ray beam away from a non-imageable area in a photographing process.
2. Related Art
Extraoral dental X-ray radiography, such as two-dimensional dental panoprex and three-dimensional computed tomography (CT), is often used for radiographing teeth, the jaw and facial bones, and peripheral soft tissues, and is a useful tool in dental diagnosis and auxiliary treatment.
An X-ray photographical contrast mainly results from penetration attenuation caused by components, density and thickness of a matter penetrated by an X-ray, and most photon energy lost in the penetration process is absorbed by human tissues and is converted into a radiation dose. Referring to FIG. 1, targets of dental X-ray radiography are mainly located below the cranium base of the skull 10 and in a range of the oral cavity and the jaw and facial bones in front of the external acoustic meatus (EAM). In skull morphology, the petrous pyramid and the midsagittal plane are boundaries of the shape of the cranium base, and a symmetrical pyramid structure with the sella turcica 11 as a vertex and having left and right angles of 47 degrees is a non-imageable area. In other words, an actual range required in dental X-ray radiography includes an area in the cranium base with the petrous pyramid and the midsagittal plane as boundaries and the sella turcica 11 as a vertex and having left and right angles of 133 degrees and a round cake column range having a height from the infraorbitomeatal line (IOML) of the skull to the mandible base. However, in a conventional disadvantageous extraoral X-ray scanning mode in the prior art, for example, dental panoprex scanning and dental CT scanning, to implement X-ray dental panoprex imaging or to obtain information of X-ray projection with an enough angle to reconstruct a three-dimensional image, most extraoral dental X-ray image systems adopt a mode of synchronized center rotation scanning in which an X-ray source and an image detecting device rotate about a center at an angle of 180 degrees for scanning.
However, since the oral cavity is located at a front and bottom position of the head and neck portion and an X-ray has a characteristic of forward advancing along a straight line, during the synchronized center rotation scanning, a non-imageable head and neck area (that is, the non-imageable area formed by a symmetrical pyramid structure area with the sella turcica 11 as the vertex and having left and right angles of 47 degrees) is often unavoidably and directly exposed to an X-ray scanning beam, resulting in extra X-ray penetration attenuation, so that the X-ray source needs a higher power to output a radiation amount sufficient for an image receiving apparatus to form an image; meanwhile, a radiation dose received by the human body is increased. If the synchronized center rotation scanning avoids scanning the non-dental imageable area such as the head and neck portion, sufficient X-ray projection image information cannot be obtained, so that a complete dental X-ray image cannot be formed.